1. Field of the Invention
This invention is related to a method of correcting relative errors in navigation systems caused by drift. More specifically, the invention relates to a self-contained drift compensation system for transmitting-receiving stations in space having duplex data links for two way communications between stations.
2. Description of the Prior Art
Two way radio frequency ranging systems are not new. Such systems have been described in Jet Propulsion Laboratory Report 32-1475 published July 1970. Ranging systems are classified in international class GO1S 3/02 and in U.S. Class 342, subclasses 112R and 451 to 458. Such ranging systems are conveniently classified or arranged by the method by coding the data being transmitted.
One class of coding systems employs pseudonoise (PN) spread sprectrum codes in the data links which permit weak spectral signals to be transmitted and decoded accurately after being received by a remote station. The use of continuous band width pseudonoise spread spectrum codes permits processing gain enhancement at the receiver and PN coding also permits highly accurate range determination between the receiver and the transmitting stations. While PN communications systems permit accurate range determination, they are not provided with accurate directional antenna structure which would permit one station in space to accurately locate the X, Y, Z, position of another station in space.
Two or more moving platforms or stations in space need to know as accurately as possible the relative position of the other station in order to properly direct their directional antenna and to also position cameras and other scientific apparatus that may be aboard the station.
The present invention has special application where the two stations communicating with each other have long periods of outage and desire to reinstate communications between each other. In such a situation it is necessary for the stations to accurately determine the position of the other in order to properly position their directional antenna and to reacquire the PN code after an outage. Each station has its own navigational system and is capable of calculating its own X, Y, Z position in space. Ordinarly, it is desirable to have the navigational systems of the individuals stations in synchronization when they are originally put into use, however, they will develop over a period of time synchronization errors due to drift. Each navigational system has its own characteristics and accuracy and thus develops its own characteristics of drift.
Modern PN ranging systems are theoretically capable of chip rates of a billion instructions per second. Using such a PN ranging system the range between stations over thousands of miles can be determined within the accuracy of one foot, thus, employing such ranging systems it is possible to synchronize one station in space from another station in space or to synchronize all stations relatively to each other. Our U.S. Pat. No. 4,357,609 issued Nov. 2, 1982 for a "Noncoherent Two Way Ranging Apparatus" sets forth the PN ranging system of the type suitable for use in the present invention.
It would be extremely desirable to provide a method of determining the location of one transmitting-receiving station relative to another station with a degree of uncertainty small enough to correct the on-board navigational system or systems of either or both station which have accumulated drift errors.